In general, inkjet printing machines or printers include at least one printhead unit that ejects drops of liquid ink onto recording media or an imaging member for later transfer to media. Different types of ink may be used in inkjet printers. In one type of inkjet printer, phase change inks are used. Phase change inks remain in the solid phase at ambient temperature, but transition to a liquid phase at an elevated temperature. The printhead unit ejects molten ink supplied to the unit onto media or an imaging member. Once the ejected ink is on media, the ink droplets quickly solidify.
The media used in both direct and offset printers may be in web form. In a web printer, a continuous supply of media, typically provided in a media roll, is entrained onto rolls that are driven by motors. The motors and rolls pull the web from the supply roll through the printer to a take-up roll. The rollers are arranged along a linear media path, and the media web moves through the printer along the media path. As the media web passes through a print zone opposite the printhead or heads of the printer, the printheads eject ink onto the web. Along the feed path, tension bars or other rolls remove slack from the web so the web remains taut without breaking.
Existing web printing systems use a registration control method to control the timing of the ink ejections onto the web as the web passes the printheads. One known registration control method that may be used to operate the printheads is the single reflex method. In the single reflex method, the rotation of a single roll at or near a printhead is monitored by an encoder. The encoder may be a mechanical or electronic device that measures the angular velocity of the roll and generates a signal corresponding to the angular velocity of the roll. The angular velocity signal is processed by a controller executing programmed instructions for implementing the single reflex method to calculate the linear velocity of the web. The controller may adjust the linear web velocity calculation by using tension measurement signals generated by one or more load cells that measure the tension on the web near the roll. The controller implementing the single reflex method is configured with input/output circuitry, memory, programmed instructions, and other electronic components to calculate the linear web velocity and to generate the firing signals for the printheads in the marking stations.
Another existing registration control method that may be used to operate the printheads in a web printing system is the double reflex method. In the double reflex method, each encoder in a pair of encoders monitors one of two different rolls. One roll is positioned on the media path prior to the web reaching the printheads and the other roll is positioned on the media path after the media web passes the printheads. A controller executing programmed instructions implements the double reflex registration method. The controller receives angular velocity signals generated by the two encoders for the two rolls, processes the signals to calculate the linear velocity of the web at each roll, and interpolates the linear velocity of the web at each of the printheads from the calculated velocities. These additional calculations enable better timing of the firing signals for the printheads in the marking stations and, consequently, improves registration of the images printed by the marking stations in the printing system.
Moving the web through the media path in a controlled manner presents challenges to web printing systems. Once such challenge occurs when a print medium moves past one or more marking stations in a print zone. As the print medium moves past each marking station, the marking station ejects ink drops onto the print medium to form images. As described above, operation of the marking stations to eject ink drops is regulated by the registration control method. Ink drops ejected from each marking station take a certain amount of time, referred to as a “flight time,” to reach the print medium. As the print medium moves past each marking station, variations in the distance between the print medium and the marking station affect the flight time of ink drops. Since the media web is in motion, often at speeds on the order of tens or hundreds of inches per second, variations in flight time of the ink drop may result in the ink drop landing on the print medium in an incorrect location, also known as a registration error. Registration errors negatively affect image quality in printed documents.
Some web printer systems position a roll, referred to as a backer roll, at a fixed distance opposite each marking station in the web printer. During an imaging operation, the print medium contacts each backer roll to ensure that the distance between the print medium and the marking station remains substantially constant. The backer rolls rotate to enable the media web to move through the print zone. Each backer roll requires fine tolerances and calibration to ensure that the media web remains at a constant distance from the corresponding marking station. The required tolerances increase the manufacturing costs of the backer rolls, and may require additional maintenance to ensure that the backer rolls remain within tolerance during operation of the printer. Consequently, improvements to printing systems that enable the use of rotating members, including backer rolls, with wider tolerances while maintaining image quality are beneficial.